An induction heater to which induction heating is applied and which is using an inverter, has an excellent heating responsiveness and controllability by being equipped with a temperature detecting element or the like in the vicinity of a thing such as a pot, which is a load, to detect the temperature of the pot or the like and adjust heating power and cooking time accordingly. Besides realizing elaborate cooking, the induction heater has the following characteristics: it hardly pollutes the air in a room because of not using an open flame; it has high heat efficiency; and it is safe and clean. In recent years, these characteristics have received attention and the demand for induction heaters has been growing rapidly.
When an object to be heated (hereinafter may be referred to as “object”) is cooked via a non-magnetic and low-resistant metal load (a container such as an pan or a frying pan made of aluminum) on the induction heater, a great floating up or buoyant force is exerted on the load by the action of a magnetic field of a heating coil on an eddy current induced in the load, and/or the load is lightweight, so that the load may move (including slipping sideways and floating) while cooking is done.
In Official Gazette of Japanese Unexamined Patent Publication 2001-332375, an induction heating cooker in accordance with the prior-art example 1 therein is disclosed which, at the start of heating, increases the heating output gradually from a low state to a set output, detects that the gradient of the change in the power source current varies to recognize the float or the movement of the load, and when the recognition is made, exercises control such as stopping the heating or lowering input power (the specific method is not described).
With reference to FIG. 56 to FIG. 60, an induction heating cooker, which is an induction heater in accordance with prior art example 2, will be described. FIG. 56 is a schematic block diagram of the induction heater in accordance with prior art example 2. FIG. 57 is a block diagram of the induction heater in accordance with prior art example 2. In FIG. 56 and FIG. 57, the numeral 110 represents an object to be heated (a metal container such as a pan or a frying pan), the numeral 101 represents an induction heating coil which produces a high-frequency magnetic field to heat the object 110, the numeral 109 represents a commercial AC power source input, the numeral 108 represents a rectifying-smoothing section comprising a bridge and a smoothing capacitor for rectifying commercial AC power source, the numeral 102 represents an inverter circuit for converting the power source rectified by the rectifying-smoothing section 108 into high-frequency power to supply a high-frequency current to the induction heating coil 101, the numeral 103 represents an output detection section for detecting the magnitude of the output of the inverter circuit 102 (specifically, a current transformer for detecting the power source current of the inverter circuit 102), the numeral 5612 represents a microcomputer, the numeral 5605 represents a setting input section having a plurality of key switches (including a key switch for inputting an output level setting command to set a target output of the induction heater), and the numeral 5601 represents a ceramic top plate which is placed on the top of a housing and on which the object 110 is to be placed. The microcomputer 5612 has a control section 5704 and a movement detection section 5706.
The movement detection section 5706 detects a movement (including slipping and floating) of the object 110 by the method which is similar to that in prior art example 1.
The control section 5704 controls the output of the inverter circuit 102 in response to an output signal from the output detection section 103 and an output signal from the movement detection section 5706. The heating output is varied by controlling the driving frequency of switching elements.
When the movement detection section 5706 does not detect the movement of the object 110, the control section 5704 exercises control so that the output (detection current) of the output detection section 103 reaches a set target current value. When the movement detection section 5706 detects the movement of the object 110, in order to stop the movement of the object 110, the control section 5704 exercises control so as to reduce the output power of the inverter circuit 102 sharply to a predetermined small value at which neither a slippage nor a float thereof is caused. Alternatively, the control section 5704 may stop the inverter circuit 102. As a result, it is possible to reduce floats and movements of the load and thereby secure the safety of the induction heater.
In FIG. 58, one example of the relationship between the input power and the buoyant force when a pan, which is an object to be heated, made of non-magnetic metal (for example, aluminum) is heated is provided. In FIG. 58, the horizontal axis indicates the input power to the inverter circuit 102, while the vertical axis indicates the buoyant force acting on the object 110. As shown in FIG. 58, the buoyant force increases as the input power grows. When this buoyant force exceeds the weight of the object, a slippage and/or a float of the object are/is caused.
The dashed line in FIG. 59 indicates a state of the change in the input current of the inverter circuit 102 until the heating output is gradually increased from a low state to a set output (target value) after startup of the inverter circuit 102 (the start of heating) and the output of the inverter circuit 102 reaches a set level of power. The solid line in FIG. 59 indicates a state of the change of the input current of the inverter circuit 102 in the case where the movement detection section 5706 detects a slippage or a float of the object 110 before the heating output is gradually increased from the low state to the set output (target value) after startup of the inverter circuit 102 (the start of heating) and the output of the inverter circuit 102 reaches the set level of power (target value). In FIG. 59, the horizontal axis indicates time, while the vertical axis indicates the input power source current of the inverter circuit 102. The induction heater in accordance with prior art example 2 shown in FIG. 59 performs the operation done after startup of the inverter circuit 102 from the beginning when the movement detection section 5706 detects a movement (a slippage or a float) of the object 110. Namely, the heating output is gradually increased until the output of the inverter circuit 102 reaches from a small output value at startup (a small output value at the start of heating) to the set output or until the movement detection section 5706 detects a movement of the object 110 again. This operation is repeated.
With reference to FIG. 60, the detection operation of the movement detection section 5706 in accordance with prior art example 2 will be described. The induction heater in accordance with prior art example 2, after startup of the inverter circuit 102 (the start of heating), gradually increases the heating output from a low state to a set output (target value) and raises the output of the inverter circuit 102 to a set level of power. Part (a) of FIG. 60 indicates the change of the input power with time in the case where a slippage or a float of the object 110 is caused before the output of the inverter circuit 102 reaches the set level of power. In part (a) of FIG. 60, the horizontal axis indicates time, while the vertical axis indicates the input power of the induction heating coil 101. Part (b) of FIG. 60 indicates the change of the power source current (the input current of the inverter circuit 102) with time in such a case. In part (b) of FIG. 60, the horizontal axis indicates time, while the vertical axis indicates the input power source current of the inverter circuit 102.
In FIG. 60, during the time the heating output is being gradually increased at the start of heating, if a buoyant force acts on the object 110, so that the object 110 moves (floats, or floats to move sideways and so on), the object 110 moves away from the induction heating coil 101. The input power of the induction heating coil 101 lowers in proportion as the object 110 moves away therefrom. When the object 110 moves, as shown in FIG. 60, the gradient of the change in the power source current (and the input power of the induction heating coil 101) gets lower. The movement detection section 5706 detects a movement of the object 110 based on the change in the gradient (time differential value) of the power source current detected by the output detection section 103.
If a user moves an object to be cooked when carrying out cooking by the use of the induction heating cooker in accordance with prior art example 2, the movement detection section erroneously determines that the object to be heated has moved by buoyant force, whereby the control section can lower the heating output or stop heating. (In prior art example 2, the operation shown in FIG. 59 is performed. In another prior art example, when a movement of the object to be heated is detected, the inverter is stopped, or the output of the inverter is limited to a predetermined low output (such a low output that a pan, of whatever kind, does not move.). In such a case, there is a problem that heating power is insufficient, whereby it is substantially impossible to carry out cooking. The induction heating cooker in accordance with prior art example 2 operates safely, but when the safety function is activated, it can be substantially impossible to carry out cooking.
The present invention intends to solve the prior art problem mentioned above and provides an induction heater having a safety function of lowering or stopping the heating power when the object to be heated moves, the safety function hardly interfering with cooking activities of the user.
The present invention provides an induction heater having the safety function of lowering or stopping the heating power when the object to be heated moves, wherein even when the safety function is activated, an induction heating coil maintains high heating power, whereby it is possible for the user to carry out cooking.
The present invention provides an induction heater which has a safety function of lowering or stopping the heating power when the object to be heated is moved by a high-frequency magnetic field produced by an induction heating coil, the safety function not being activated in any case other than mentioned above so that the situation where cooking activities of a user are hindered by the safety function is prevented.
The present invention provides an induction heater having the safety function of lowering or stopping the heating power when the object to be heated moves, wherein the safety function is not activated when a user moves a pan which is the object to be heated, or even when the safety function is activated, it is possible to heat the object with stability (for example, it is possible to carry out the cooking such as frying).